Abstract : Capabilities in Flex are first class data objects which allow one to define and limit the right to access data or obey an action. Their use extends from mainstores to filestores and across networks of Flexes. This paper gives a general description of how Flex capabilities are implemented, controlled and used. They are classified into four varieties, mainstore, filestore, remote and universal. Each of these varieties has its own range and lifetime designed to combine consistency, integrity and utility with implementability. (Author)